Organic light emitting diode (OLED) display devices are becoming attractive due to their advantages such as low power consumption, high lightness, low cost, wide viewing angle and quick response speed and so on, and are widely used in the organic light emitting field.
In OLED display devices, there are following problems. Firstly, transistors for achieving image display on a backplane lack of uniformity in their structures, electrical properties and stability, which causes the threshold voltages Vth shifts of transistors. Secondly, in a case where the transistors are turned on for a long period of time, the stability of the transistors decreases. In addition, as OLEDs are increased by size during development, loads on signal lines become large, which results in a voltage attenuation on the signal lines, such as changes of operation voltages.
When the OLED is driven to work by a conventional pixel circuit for driving the OLED to emit light, current flowing through the OLED depends on at least one of factors including the threshold voltage Vth of the driving transistors, the stability of driving transistors and a reference voltage Vref. When each pixel is applied by a same driving signal, in a displaying region of the backplane, the currents flowing through the OLEDs are not equal to each other, which results in the non-uniformity of the currents on the backplane and thus the non-uniformity of the brightness in the image.